1. Field of the Invention
This invention relates to the field of reinforced plastic pipes. More specifically, the invention relates to a butt-wrap (or butt-strap) joint between lengths of reinforced plastic pipes.
2. Description of the Related Art
GRP pipes are commonly joined by xe2x80x98butt-wrapxe2x80x99 (or xe2x80x98butt-strapxe2x80x99) joints, i.e. in an end-to-end (or edge-to-edge) configuration and wrapped with one or more wraps of fabric. The resulting laminate generally covers equal amounts of the two abutting pipe ends. The laminate typically consists of several wraps of reinforcement mats and/or woven rovings saturated with a thermosetting resin or other agent. The resin is cured by application of heat or by chemical means, thus changing the laminate into a substantially infusible and insoluble material (thermoset plastic). When curing, the laminate bonds with the pipe end outer walls, thus forming a joint with sufficient strenght in both the circumferential (hoop) and longitudinal directions.
Current butt-wrap technologies are based on the use of laminates with traditional reinforcements in the form of mats and/or woven roving with equal strength and stiffness properties in both hoop and longitudinal directions. Prior art joints are designed so as to be able to withstand the design loads in both hoop and longitudinal direction, independent of the pipe itself, thus disregarding important aspects like the pipe""s load carrying capacity in the hoop direction and stress concentrations generated by discontinuity.
The minimum bond length (the longitudinal dimension of the laminates on each pipe end) is typically determined based on simplified models for shear stress distribution in the bonding surface between the pipe and the laminate. In fact, the bond length is normally determined such that the average shear stress over the bonding surface shall not exceed a fixed value, independent of the real shear stress distribution and how it is influenced by factors like the stiffness ratio between pipe and wraps, absolute bond length, geometric and elastic properties and stress concentrations. Radial stresses and their influence on bonding strength as well as discontinuity stresses are commonly disregarded.
As a consequence, the butt-wraps currently in use tend to be overly thick and rigid giving rise to severe stress concentrations in the pipes joined, negatively affecting their performance. The simplified models for lap shear strength do in many cases correlate poorly to the real strength, which may lead to either overly conservative and costly butt-wraps, orxe2x80x94in the worst casesxe2x80x94to underdesign.
It is therefore a long felt need for an improved butt-wrap joint for reinforced plastic pipes, that have optimal design characterisics and is less time consuming and less costly to produce.
The present invention solves that need, in that it provides a novel butt-wrap joint for fiber reinforced thermosetting resin pipes, said joint being structurally and economically improved compared to the prior art. In this regard, the main focus is on improved structural reliability of the joint, increased load carrying capacity, improved utilisation of the pipes structural capacity through reduction of stress concentrations and reduced cost of the joint.
The above objective is attained by the aid of thorough structural analysis of the pipe together with the wraps and the use of efficient reinforcement fabrics and resins for the wraps.
Structural analyses and tests performed by the applicant have revealed that the longitudinal loading capacity of the butt-wrap joint, rather than the hoop strength, is the critical design factor. In that context, it is apparent the shear- and radial stress concentrations occurring in the bonding surfaces at the end of the pipe and the wraps, as well as the longitudinal stress concentrations acting in the pipe wall adjacent to the wraps, are the most important load responses that have to be controlled by proper design of the wraps.
The structural analyses and testing have also shown that the bonding stress concentrations are most efficiently controlled by:
1) balancing of the longitudinal stiffness of the wraps against the longitudinal stiffness of the pipe,
2) tapering of the wraps and/or the pipe spigot, and
3) optimizing the geometric-elastic properties of the wraps and the pipe.
Referring to step 2 above, a tapering of the spigot is highly impractical and will normally not be performed, unless specific conditions (e.g. unbalanced laminate) require it. A much more efficient approach is to displace the shear stress to the laminate edge by increasing the laminate longitudinal stiffness.
The radial stresses are most efficiently controlled by minimising the eccentricity of the wraps in the longitudinal direction. The longitudinal stress concentrations in the pipe are, on the other hand, most efficiently reduced by smooth tapering of the wraps and by minimising the hoop stiffness of the wraps.
The structural capacity and efficiency of butt-wrap joints can also be increased by correct choice of resin and reinforcement. Resins with better adhesion will allow for higher shear and radial stress concentrations in the bonding surface. Because of the limitations of secondary bonding strength compared to the strength of cured in one resin, it is under most circumstances beneficial to use a higher quality resin in the wraps than in the pipes.
Non-woven fiber reinforcements are also beneficial compared to woven reinforcements. With straight fibers the radial stress, generated when the undulated fiber of woven roving are tensioned, are avoided and with the higher degrees of reinforcement achieved, curing stresses are reduced.
The butt-wrap joint according to the invention generally comprises a plurality of wraps of material wetted with a thermosetting resin and wrapped circumferentially relative to a first pipe end and a second pipe end, where the first and second pipe ends are the respective ends of first and second pipes. The pipes are aligned and the two pipe ends are abutting. The pipes are being joined at their respective abutting ends by the wraps disposed generally symmetrically on said abutting pipes. The first wrap is disposed relative to the respective pipes and second through n-th wrap are disposed successively outwardly, relative of the respective preceding wrap. The first wrap has a width L1, the second wrap has a width L2, and the n-th wrap has a width Ln, all in in the pipe longitudinal direction. The wraps are covered by a chopped strand mat (CSM) wetted with the thermosetting resin and having a width L0 in the pipe longitudinal direction. The butt-wrap joint in accordance with the invention is is thus characterised in that:
a first wrap width L1 is greater than the width of any of the subsequent wraps;
a second wrap width L2 is greater than the width of any of the subsequent wraps;
the widths L3 to Lnxe2x88x921 of the intermediate wraps are successively diminishing at a constant rate (2xcex3), where this rate is a result of a linear interpolation between the second wrap L2 width and the n-th wrap width Ln;
the longitudinal stiffness (Elongxc3x97t) of the wraps is progressively tapered at the edges, in that the longitudinal stiffness of the wraps is increased at a low rate (wrap stiffness vs distance from wrap edge) at the edge of the wraps while it is increased at a faster rate away from the edge.
Preferred embodiments of the invention are that: The rate of increase in wrap stiffness is preferably increased in 3 or more steps. The variable n may be any number greater than or equal to 3. The laminate has a substantially higher tensile stiffness in the longitudinal direction than in the circumferential direction. As each of the wraps has a tensile stiffness in the longitudinal direction which is substantially (e.g. 50%-200%) higher than the tensile stiffness in the circumferential direction, and said chopped strand mat (CSM) has generally uniform tensile stiffness in both the longitudinal and circumferential directions, the resulting laminate has a tensile stiffness in the longitudinal direction which is substantially higher than the tensile stiffness in the circumferential direction. The resin is a vinylester resin, or (preferably) a bisphenol A vinylester resin. The butt-wrap resin may be a vinyl ester while the pipe resin is a polyester. The butt-wrap resin and the pipe resin may be different resins. The butt-wrap resin may have a higher quality than the pipe resin.
The longitudinal tensile stiffness of the laminate is greater than longitudinal tensile stiffness of the pipes. The wraps are comprised of non-woven roving fabric. The wraps are disposed externally relative to said pipes. The wraps are disposed internally relative to said pipes. The wraps are disposed both externally and internally relative to said pipes. The resin is in a cured (or set) state and/or in a non-cured (or set) state. The pipes are comprised of glass fibre reinforced plastic, carbon fibre reinforced plastic, or other types of fibres.